The present invention relates to distillation systems for liquid treatment processes. In particular, the present invention relates to distillation systems containing hollow fiber membrane modules for separating distillate fluids from feed solutions in liquid treatment processes.
In recent years, membrane distillation has become increasingly popular in a variety of fluid-treatment applications. The membranes are typically hydrophobic and microporous to keep the feed solution separated from that of the distillate during operation. Hollow fiber membranes are typically employed in tube/shell configurations, where bundles of hollow fiber membranes are arranged along the longitudinal axes of the modules. For membrane distillation, each hollow fiber membrane in the bundle is typically a hydrophobic, microporous membrane having an exterior surface and an inner hollow tubular region. The exterior surfaces of the hollow fiber membranes face a shell side of the module, which is the portion of the module containing the feed solution. The inner hollow tubular regions define a tube side of the module, which provides a conduit for collecting the distillate fluids separated from the feed solution.
During operation, the feed solution is typically heated to form a temperature differential across the wall of the hollow fiber membranes. This temperature differential creates a vapor pressure differential between the tube side and the shell side of the module, which causes vapor transport through the hollow fiber membranes. The transmitted vapor then condenses within the hollow fiber membranes, thereby providing the desired distillate fluid.
During steady state operations, the mass transfer across the hollow fiber membranes can provide good product rates of distillate fluids for a variety of distillation systems. However, a common issue with hollow fiber membrane modules involves the thermal efficiencies of the modules. As the distillate fluid evaporates and transmits through the hollow fiber membranes, latent heat accompanies the transmitted vapor, passing from the shell side to the tube side of the module. Furthermore, the temperature differential between the shell side and the tube side of the module can result in conductive heat transfers across the wall of the hollow fiber membranes. Overall, a large amount of heat is transported through the membrane. If the heat is not recuperated, a high operating cost in terms of thermal energy would result. Thus there is an ongoing need for increased thermal efficiencies in distillation systems containing hollow fiber membrane modules.